Do you want to know how many user accounts/objects you can create in active directory? Want to know how many members a group can have? Want to know maximum number of GPOs a domain can have?. Microsoft AD documentation team has setup a technet page about these details along with some more valuable information.
Link to document : http://technet.microsoft.com/en-us/library/active-directory-maximum-limits-scalability(WS.10).aspx